1. Field of the Invention
This invention pertains to the determination of total organic carbon in an aqueous sample containing halide ion, e.g., seawater.
2. State of the Prior Art
A technique for determining total organic carbon (TOC) in a seawater sample was described in an article by M. Erhardt entitled "A New Method for the Automatic Measurement of Dissolved Organic Carbon in Sea Water" published in Deep-Sea Research, Vol. 16, pages 393-397 (1969), in which dissolved organic matter in the sample is oxidized with a persulfate oxidizing agent using ultraviolet radiation to promote the oxidation, and in which carbon dioxide formed by oxidation of the organic matter is quantitatively detected to determine total carbon.
TOC in an aqueous sample containing dissolved organic matter has been determined by irradiating the sample with ultraviolet radiation from a low-pressure mercury vapor lamp immersed in the sample in order to oxidize the organic matter to carbon dioxide, sparging the carbon dioxide so produced from the reactor, and then measuring total carbon in the sparged carbon dioxide. Such a technique was described in an article by P. Wolfel and H. Sontheimer entitled "Ein neues Verfahren zur Bestimmung von organisch gebundenem Kohlenstoff im Wasser durch photochemische Oxidation" published in Vom Wasser, Vol. 43, pages 315-325 (1974).
In co-pending U.S. patent application Ser. No. 127,333, a technique was described for determining total carbon in each one of a plurality of liquid samples by maintaining a continuous flow of a liquid containing an oxidizing agent (e.g., persulfate ion S.sub.2 O.sub.8.sup.-- or hydroxy free radical .OH) through a reactor into which the samples are introduced in succession, and by irradiating each sample in the reactor with ultraviolet radiation from a mercury vapor lamp immersed in the sample to oxidize carbonaceous matter in the sample and thereby produce carbon dioxide that can be measured for total carbon.
In chemical oxygen demand (COD) analysis, where dichromate ion is used as the oxidizing agent, free chloride ion Cl.sup.- in a sample would react with the oxidizing agent, thereby decreasing the amount of oxidizing agent available for the analysis. It was known, however, that by adding mercuric sulfate HgSO.sub.4 to the sample, free Cl.sup.- would be complexed with Hg.sup.++ to form mercuric chloride HgCl.sub.2, thereby removing free Cl.sup.- from interfering with the analysis. However, adding HgSO.sub.4 to an aqueous sample containing persulfate ion S.sub.2 O.sub.8.sup.-- produces an insoluble precipitate, i.e., mercuric oxide HgO, which cannot react with chloride ion Cl.sup.-.
It was known that mercuric monochloride ion HgCl.sup.+ can be formed by mixing equal molar concentrations of mercuric chloride HgCl.sub.2 and mercuric nitrate Hg(NO.sub.3).sub.2 together in aqueous solution. However, HgCl.sup.+ is not usually used to complex free chloride ion Cl.sup.- for analytical purposes. Use of a solution containing HgCl.sup.+ for complexing free chloride ion Cl.sup.- appears to have had no practical application until the present invention.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide a technique for determining total organic carbon in an aqueous sample containing halide ion using a reagent that oxidizes organic matter in the sample in the presence of ultraviolet radiation, where the rate of oxidation of the organic matter is not significantly suppressed due to halide ion interference.
It is a particular object of the present invention to provide a technique for determining total organic carbon in an aqueous sample containing halide ion by ultraviolet-promoted oxidation of organic matter in the sample using persulfate ion as an oxidizing agent, where suppression of the rate of oxidation of the organic matter due to halide ion interference is minimal.
More specifically, it is an object of the present invention to provide a reagent for use in oxidizing organic matter in an aqueous sample containing halide ion in the presence of ultraviolet radiation, where the reagent contains persulfate ion and mercuric monochloride ion.
It is likewise an object of the present invention to provide a precipitate-free reagent containing persulfate ion and mercuric monochloride ion for use in ultraviolet-promoted oxidation of organic matter in an aqueous sample containing halide ion.